1. Field of the Invention
This invention related to apparatus for measuring the electrical impedance of materials, and in particular relates to a contoured electrode, a part of the apparatus for measuring the electrical impedance of material, shaped to accommodate different types of sample material in order that the electrical field radiated by the electrode passes through a sufficient portion of material sample enhancing the accuracy of the impedance measurement.
2. Description of the Prior Art
Optimizing the yield of various manufacturing processes often requires very accurate monitoring of the moisture content of a given material. For example, the milling of wheat is carried on most efficiently when the wheat has a 15% moisture content. During the pulping of wood chips, the moisture content of the wood chips must be known in order to determine the proper amounts of liquor necessary to be added for maximum delignification. The required accuracy for the measurement of moisture varies for different materials and is shown below for some representative materials:
______________________________________ Typical Accuracy Material Moisture Range of Moisture Reading ______________________________________ Wood Chips 30%-70% .+-.2% Wheat 8%-15% .+-.0.2% Paper 5%-10% .+-.0.5% Plywood Veneer 3%-8% .+-.0.5% Potato Chips 12%-17% .+-.0.5% Lumber 30%-70% .+-.3% Tissue paper 3%-10% .+-.0.5% ______________________________________
The disclosures in the above-identified related patents should be consulted to put the instant invention in perspective, and for an in-depth description of the component parts of a moisture detection system.
In order to make electrical impedance measurements of a sample material with the degree of accuracy necessary to determine moisture content, it has been found that it is desirable to construct the electrode to accommodate the different characteristics of the material to be sampled. For example, particulate materials of relatively uniform size and shape, such as grain or the like, are normally free flowing and are not subject to blocking when flowing through an area confined by an impedance measuring electrode. Other materials, such as corn fibers, potato chips and wood chips, are not nearly so free flowing, and it has been found that additional handling systems are generally desirable to pack the material and to transport it past the impedance measuring electrode.
The flow problem associated with the material to be sampled is particularly acute with wood chips, since, unlike grain, the size and shape variations in the chips tend to cause "bridging" as they pass the impedance measuring electrode. Sticky wood chips, such as those freshly cut during the early summer months, have an even greater then normal tendency to "bridge" or lodge in a sample box which also functions as an impedance measuring electrode.
A more exhaustive discussion of the problems associated with wood chips moisture measuring apparatus may be found in an article entitled "Factors Affecting Automatic Wood Chip Moisture Measurement" by F. K. Preikschat, Ekhard Preikschat and Daniel F. Pope, published in Paper Trade Journal dated July 29, 1974. This same article also describes a sample box suited for use with wood chips including a clam shell door at the lower end of the sample box.
In some manufacturing operations, such as pulp manufacturing, the bulk density measurement of the wood chips can be as important as a determination of the moisture content. Regardless of what type of digester is used in the pulping operation, the wood chips often are initially measured on a volumetric basis which means that in order to determine the total weight of chips entered into the digester the bulk density must be known. If both the moisture content and the bulk density of the wood chips in the material are known, these factors can then be used to determine the correct amount of liquor that is needed for the digester process itself.
In the digester process, it turns out that the wood to liquor ratio is one of the most important control parameters. The reason for this is that in order to obtain an optimum delignification, i.e. a dissolution of the lignin bonding the wood fibers, a certain amount of liquor has to be added to a given amount of wood chips to obtain an optimum delignification with minimum destruction to the wood fiber itself. Typically, the moisture content of the wood chips can vary over a 30% moisture range which is typically 35% to 65%. By the same token, the bulk density can also vary tremendously as determined by the size distribution of the wood chips, wood species, the type of chips used, e.g. core wood or sap wood, and the type of growth conditions encountered. Since there are so many variables involved, once the wet weight bulk density is related to the moisture content of the wood chips, it has been found that the bulk density may still vary over a considerable range. For example, under Southern growth conditions, it has been found that even though the moisture content does not vary by more than 4%, the bulk density can still vary by more than .+-. 15%.
Furthermore, wood chips are not homogeneous and even those in the same pile can differ widely in their physical makeup. Chips are produced from individual trees, which themselves are trucked to a common point from a wide surrounding area. If the moisture and bulk density of the chips contained in a storage pile were measured, both the moisture and bulk density would vary in a random fashion. Most often these parameters follow a normal distribution curve having an average deviation in moisture of .+-. 15% and an average deviation in bulk density of about .+-. 25%.
In the case of a continuous digester, wood chips and liquor are added in predetermined amounts simultaneously and continuously so as to produce a pulp of a certain quality. Here again, wood chips are added on a volumetric basis, as measured by the revolutions of the star feeding valve. The star feeding valve has a certain number of pockets, each with a given volume which makes it possible to readily calculate the volumetric feeding rate. If the wet weight bulk density of the wood chips is known, the volumetric feeding rate of the star valve can readily be translated into a wet weight flow of chips. This, in conjunction with the known or measured moisture content, can be translated into a bone dry weight flow of chips.
A more detailed discussion of pulp processing methods and the effects of variations in moisture content and bulk density may be found in an article entitled "Comparison of Pulping Methods" by Burton E. Helberg et al., published in TAPPI, Vol. 59, No. 5, May 1976.
In some manufacturing, a moisture control test may be used to ensure quality of an end product by measuring the moisture content to determine if it is within predetermined limits. For example, it is important that facial tissue when shipped from the manufacturer have a moisture content which does not exceed predetermined limits. At present, the moisture content of facial tissue is normally determined on a spot check basis. However, it would be desirable if all rolls or stacks of tissue paper could be checked for moisture content on a continuous basis prior to packaging and shipping without adding significantly to production cost.
Still another application where it is desirable to accurately measure moisture content is in the drying of lumber. One method of drying lumber involves positioning the rough cut wood in a closed heated structure, such as a kiln or the like, for a period of from 1 to 5 days. The drying rate is not a constant but depends on a number of variables such as the type of lumber, initial moisture content, temperature within the kiln, etc. Presently there is no reliable way to measure the drying rate or to determine the point during the drying cycle at which a desired amount of moisture has been removed from the wood. A more precise and accurate measurement of moisture content during this drying cycle is important for several reasons. First, the cost of energy and heating in recent years has increased dramatically and it has become economically imperative that the drying time be kept to a minimum. Second, when wood is overdried it becomes very difficult to work with wood working machines, such as lathes and planers, further increasing lumber production costs. Under current practice, if lumber has been overdried, it is often rewetted in order to raise its moisture content to approximately 10%.
One known method of ascertaining dryness presently used by kiln operators involves manual spot checking of the moisture content of indivdual pieces of lumber in the kiln. This procedure involves the use of a two-prong resistance gauge which is pressed against the surface of a piece of lumber to determine surface resistance. This resistance reading is then related to moisture content via a chart or formula. Inaccuracies are inherent in this method since the resistance reading on the gauge can vary depending on the amount of contact pressure, the temperature of the wood, the surface properties of the wood, and its position in the stack, in addition to the moisture content of the lumber itself.